Copper base alloys containing iron and aluminum



United States Patent 6 COPPER BASE ALLOYS CONTAINING IRON AND ALUMINUM Cyril H. Hannon, Pittsfield, Mass., assignor to General Electric Company, a corporation of New York No Drawing. Application November 12, 1954,

Serial No. 468,577

7 Claims. (Cl. 75-159) The present invention relates to improved alloys of the copper-nickel-silicon type.

Copper-base alloys of the above type, while highly desirable for many industrial applications, are generally susceptible to a type of defect known in metallurgical arts as stress corrosion. The term does not'refer merely to a combination of stress and ordinary corro- 'sion. Ordinary corrosion is generally considered to be chemical corrosion, the corrosive medium chemically dissolving the metal on which it acts, resulting in a gradual thinning of the metal. Thekind of corrosion met with in stress-corrosion is electro-chemical corrosion caused by the existence in the alloy structure of more or less continuous zones that are anodic to contiguous portions of the alloy. The presence of such anodic portions causes a corrosive. medium to set up a galvanic cell in which such medium is the electrolyte and the contiguous portions of different electro-chemical potential are the electrodes, the anodic zone or portion dissolving into such electrolyte by galvanic action, with the result that the surface of the alloy subjected to the medium becomes locally pitted as distinguished from being gradually thinned by chemical solution.

When the surface of an alloy having these contiguous relatively anodic and cathodic portions is subjected to the action of a corrosive medium, minute fissures or pits are initiated at such surface due to this electro-chemical or galvanic action, and when residual stress from prior mechanical working and/or high tensile stresses resulting from tensile or torque loading of the article made of the alloy are present, these small fissures increase the magnitude of local stresses at the fissures and that increases the depth of the fissures. This increase in the depth of the fissures exposes fresh metal at the bottoms of the fissures, the thus exposed fresh metal being consumed by the electrolytic action with consequent further increase in the magnitude of the stresses at the fissures,

and as the action progresses the alloy ultimately cracks or otherwise fails. The stress-corrosion asthus described leads ultimately to a form of rupture called stress-corrosion cracking sometimes also referred to as season cracking.

It is an object of the present invention to provide a copper-base alloy of the abovetype which is not susceptible to stress-corrosion cracking or season cracking failure (associated therewith) and which is characterized by high tensile strength, resistance to chemical corrosion and oxidation and by improved working behavior.

In the copending application of Briggs and Hannon, Serial No. 235,898, filed July 9, 195l, now Patent No. 2,744,822, dated May 8, 1956, there are described alloys which are resistant to stress-corrosion failure. The copending application provides for the elimination of susceptibility of the copper-nickel-silicon type alloy to stresscorrosion cracking by in substance incorporating into the alloy a critical amount of iron within narrow critical ranges of nickel and silicon, the resulting lack of susceptibility of the improved alloy to failure by stresscorrosion being caused by the absence in the alloy of the above-mentioned contiguous relatively anodic and cathodic portions. The amount of iron required in the ice (alloy disclosed in the copending .application is in the range of 0.3 to 1%, it being indicated therein that amounts of iron above that range will result in an increase in susceptibility of the alloy to chemical and electro-chemical corrosion, will make it ditficult to machine and not possible to work satisfactorily without cracking, While amounts of iron below that range provide an alloy having a very poor resistance to stress-cor- 'rosion and one which has an unsatisfactory low yield point. The copending application further cites detrimental eifect-s on the alloy by the addition of aluminum, in that the aluminum makes the alloy less resistant to 'season cracking and more difficult to machine.

It has been found, however, in accordance with the present invention that aluminum may be added to the 'alloy composition without causing the above-mentioned detrimental effects, if an amount of iron within a particularly critical range is incorporated in the alloy. The resulting alloy is thereby characterized by high tensile strength, resistance to surface and sub-scale oxidation, resistance to stress-corrosion and ease of machining and working.

-In accordance .with the invention, the copper-base alloy contains approximately 3.5 to 5% nickel, 0.7 to 2% silicon, between 3 and 10% aluminum, a critical i-ron content of 1.5 to 5%, and the balance being copper. While the aluminum may be used in amounts of between 3 and 10%, a preferred amount is approximately 5%. The

alloys of the above composition may be hot forged, hot or cold rolled or drawn, and hot extruded to form various shapes. Furthermore, these improved alloys are very resistant to chemical corrosion and season cracking, and have the valuable property of being age hardenable by suitable heat treatment. The tensile and yield strengths of the improved alloys are particularly high, and it is in the improved tensile strength and oxidation resistance, due principally to the aluminum content, that the present alloy composition differs notably from, and is a considerable improvement over, the alloy disclosed in the copending application.

The proportion of the components in the alloy compi sition may be varied within the above ranges depending on the particular uses in mind or the results sought. For example, the silicon may be present in amounts of from 1.6 to 2%, the iron from 3.5 to 5% and the aluminum from 7 to 10% where best results are sought in the alloy as cast.

The importance of the particular iron content of the present alloy is demonstrated by the. fact that alloys containing 3.5 to 5% nickel, 0.7 to 2% silicon, nominal 5% aluminum and less than 1.5% iron, with the balance of copper, are susceptible to stress-corrosion failure. the hot and cold working behavior become impaired to the extent that the alloys are no longer practically workable, as evidenced by excessive tool or die wear. Furthermore, above about 5% iron the machinability of the alloys is markedly impaired, resulting in extremely short tool life.

The following compositions are examples of alloys which have been found practically useful in resisting stress-corrosion fracture when tested at torque stress loads equal to of the ultimate strength, the values being in percent by weight:

Alloy A Alloy B Alloy O 4.6 4. 5 4. 6 95 71 76 2.0 2.9 4. 7 4. 9 5. 3 5.0 bal. bal. be].

If the iron in these alloys exceeds about 5% both.

Alloys of the same basic composition. as those above but containing less than 1.5% iron failed by cracking when tested under comparable conditions to reveal sus- 'ceptibility to stress corrosion. The evaluation tests were made on screw threaded studs of /s diameter having '16 U. S. standard screw threads per inch, the .studs 'beingassembled in steel fixtures and clamped by nuts. on their threaded portions so as to apply measured torque loads. With the torque load of 95% ultimate strength applied to compared studs having a different iron content as mentioned above, the stressed studs were immersed in a mercurous nitrate solution (ASTM Standard) for. 15 minutes. -Whereas ,the alloy specimens of less than 1.5% iron content all cracked within the specitied 15 minutes, none of the alloys of the present composition cracked during that immersion period. The unercurous nitrate solutionwas selected as the corrosive medium in these tests because of its property of vigorously'attacking stressed copper-base alloys of high copper content. of accelerated life test, since in a few minutes the *mercurous nitrate solution will produce the corrosive effect on the alloy that it would take an industrial atmosphere years to produce. It is also to be noted that screw threaded rods of small diameter were selected for the test, and those rods subjected to torque loading in making the tests, because-the notch effect of the screw threads on rods of small diameter and the twisting efiect on such rods produced by such loading present the optimum conditions for failure by stress corrosion when the rods are subjected to the action of a corrosive medium.

In order for the improved alloy compositions to have \the desired properties theygshould contain a minimum -amount of nickel silicide. It has been found that if less than 3.5% nickel is present in the alloy, and the silicon is within the range thereof specified, suflicient nickel silicide will not be formed and the alloy will 'lack the desired properties, particularly that of effectively resisting stress corrosion, even if there is chemically uncombined nickel and silicon in the alloy. Within the specified ranges of 3.5 to nickel and 0.7 to 2% silicon, sufiicient nickel silicide apparently will be formed, and the addition of about 1.5 to 5% .iron will give the alloy its property of-highly resisting. stress-corrosion cracking where the alloy also contains an amount of aluminum in the range specified above. Where the al- 'loy contains less thanabout 1.5% iron within the specified ranges of 3.5 to 5% nickel and 0.7 to 2% silicon,

it will make the alloy unsatisfactory for therea-son that lthe presence of aluminum under those conditions will make the alloys difficult to machine and subject toseason cracking.

.The alloys formed of the present composition arecharacterized by relatively high tensile strength properties,

The corrosion test thus described is a type 123,000 p. s. i. tensile strength of the present alloys annealed and aged as compared to 113,000 p. s. i. of a similarly treated alloy of the copending application.

Alloys within the scope of the present invention are also characterized by the formation, due to the inclusion 'of the specified amount of aluminum, of thin adherent oxide films during normal heating or annealing cycles. These tenacious thin films retard progressive oxidation 'and'etfectively prevent the formation of what is known by the technical term of sub-scale. This characteristic allows hot fabricating operations on the alloys byheating in normal air atmosphere. Special protective atmospheres or fused salt baths are therefore not essential for the heating of the alloys.

The properties of resistance to stress-corrosion cracking and high tensile strength which characterize the'present alloys make them particularly suitable for use in the manufacture of electrical cable pressure terminals, fixtures for power transmission lines, turnbuckle and other eyebolts, and other articles which are subjected to particularly high tensile loads under the corrosive conditions of industrial atmospheres.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An alloy characterized by high resistance to oxidation and stress-corrosion failure and high tensile strength comprising approximately 3.5 to 5% nickel, 0.7 to 2% silicon, 3 to 10% aluminum, 1.5 to 5% iron, and the balance copper.

2. An,alloy characterized by high resistance to oxidationnand stress-corrosion failure and high tensile strength-comprising approximately 3.5 to 5% nickel, 0.7 to 2% silicon, 5% aluminum, 1.5 to'5% iron,- and the balance copper.

3. An alloy characterized by high resistance to oxida- -tion-and stress-corrosionfailure and high tensile strength comprising approximately 4% nickel, 1% silicon,'2%

iron, 5% aluminum, and the balance copper.

'4. A wrought article characterized by high resistance to oxidationand stress-corrosion failure and high tensile strength, saidarticlev being made of an alloy composed of approximately 3.5 to 5% nickel, 0.7 to 2% silicon, 3 to 10% aluminum, 1.5 to 5% iron, and the balance copper.

5. An. alloy characterizedby high resistance to oxidationand stress-corrosion failure and high tensile strength comprising approximately 3.5 to 5% nickel, 1.6 to 2% silicon, 3 to 10% aluminum, 1.5 to 5% iron,v and the balance .copper.

6. An. alloy. characterized by high resistance to oxidation andstress-corrosion failure and high tensile strength comprising approximately 3.5 to 5% nickel, 0.7 to 2% silicon, 3 to 10% aluminum, 3.5 to 5% iron, and the balance copper.

' 7*. Anqalloy characterized by high resistance to oxidation and stress-corrosion failure and high tensile strength comprising approximately 3.5 to 5% nickel, 0.7 to'2% silicon, 7 to 10% .-aluminum, 3.5 to 5% iron, and the balance copper.

;;References Cited in-the file of this. patent UNITED STATES PATENTS 2,031,315 Jennison -Feb. 18, 1 936 

1. AN ALLOY CHARACTERIZED BY HIGH RESISTANCE TO OXIDATION AND STRESS-CORROSION FAILURE AND HIGH TENSILE STRENGTH COMPRISING APPROXIMATELY 3.5 TO 5% NICKLE, 0.7 TO 2% SILSCON, 3 TO 10% ALUMINUM, 1.5 TO 5% IRON, AND THE BALANCE COPPER. 